1. Field of the Invention
This invention relates to a balanced transformer-less amplifier circuit and, more particularly, to a balanced transformer-less amplifier circuit suitable for battery operation.
2. Description of the Prior Art
Balanced transformer-less amplifier circuits are able to supply a relatively large power output with a low supply voltage. Therefore, the balanced transformer-less amplifier circuits are widely used as power amplifiers of battery operated equipment, such as portable radios, portable tape players/recorders and the like. The balanced transformer-less amplifier circuits have a preamplifier circuit and a pair of power amplifier circuits which are coupled to the output terminal of the preamplifier circuit in parallel with each other. The pair of power amplifier circuits are so designed that their outputs have opposite polarities to each other. That is, one of the power amplifiers operates as a non-inversed output amplifier, while the other power amplifier operates as an inversed output amplifier.
Recently, amplifiers including the balanced transformer-less amplifier circuits have been fabricated into forms of integrated circuits (referred as ICs hereafter). As is well known, the ICs are required to have capacitors as low a capacitance as possible, since the capacitors occupy a huge area on the IC chips. On the other hand, loads such as loudspeakers generally are coupled to amplifiers therefor, via suitable filter circuits for preventing DC (direct current) currents from flowing through the loads. The filter circuits conventionally are comprised of capacitors with large capacitance. Therefore, the amplifiers generally are fabricated into the IC configurations without including the coupling capacitors therein. Then the loads are coupled to the amplifiers of the IC forms, via coupling capacitors outside the amplifiers in the IC forms.
The balanced transformer-less amplifier circuits are suitable for the amplifiers in the IC forms for driving these loads. This is because the balanced transformer-less amplifier circuits are able to be coupled to the loads without using the coupling capacitors. For the conventional balanced transformer-less amplifier circuits, a reference is made to the non-examined Japanese Patent Publication No. 53-129569.
Although the above mentioned Patent Publication fails to disclose all the details of the balanced transformer-less amplifier circuit, it is well known that this type of the balanced transformer-less amplifier circuit is driven by a power supply circuit with a positive and a negative voltage source. The positive and negative voltage sources make it possible for both of the DC output levels of the pair of power amplifiers to be kept at the same levels of the zero potential. As a result, loads therefor can be coupled between the output terminals of the pair of power amplifiers without using coupling capacitors.
FIG. 1 shows an example of the conventional balanced transformer-less amplifier circuits corresponding to the above-referenced Patent Publication. In FIG. 1, a balanced transformer-less amplifier circuit 10 has a preamplifier 11, a pair of power amplifiers, i.e., first and second power amplifiers 12, 13 and a circuit 14 for detecting an offset output voltage, as described later. The preamplifier 11 is constituted by first and second PNP transistors 15, 16. The emitter-collector paths of the first and second PNP transistors 15, 16 are connected between first and second power supply terminals 17, 18 in parallel with each other. The first and second power supply terminals 17, 18 are adapted for receiving prescribed positive and negative voltages +Vcc, -Vee, respectively.
The emitters of the first and second PNP transistors 15, 16 are connected to the positive power supply terminal 17 through current sources 19, 20, respectively. The collectors of the first and second PNP transistors 15, 16 are connected to the negative power supply terminal 18 through collector load resistors 21, 22, respectively. The base of the first PNP transistor 15 is coupled to an input signal reception terminal 23. The input reception terminal 23 is adapted for receiving an input signal to be amplified in the balanced transformer-less amplifier circuit 10. The base of the second PNP transistor 16 is coupled to the output terminal of the offset output voltage detecting circuit 14. Further the bases of the first and second PNP transistors 15, 16 are connected to a ground terminal 24 through base bias resistors 25, 26, respectively. The ground terminal 24 is adapted for a connection to a ground potential source 27.
The first and second power amplifiers 12, 13 are constituted by non-inversed amplifiers, respectively. The input terminals of the first and second power amplifiers 12, 13 are coupled to the collectors of the second and first PNP transistors 16, 15, respectively. The output terminals of the first and second power amplifiers 12, 13 are coupled to first and second output signal supply terminals 28, 29 of the balanced transformer-less amplifier circuit 10. The first and second output signal supply terminals 28, 29 are adapted for supplying a prescribed load such as a loudspeaker 30 with the output of the balanced transformer-less amplifier circuit 10, i.e., a differential output between the first and second output signal supply terminals 28, 29. The load 30 is coupled between the first and second output signal supply terminals 28, 29. The output terminals of the first and second power amplifiers 12, 13 are further coupled to the emitters of the first and second PNP transistors 15, 16 through feedback resistors 31, 32, respectively. Therefore, the input terminal of the first power amplifier 12 is connected to the second PNP transistor 16, but its output terminal is connected to the first PNP transistor 15. The input terminal of the second power amplifier 13 is connected to the first PNP transistor 15, but its output terminal is connected to the second PNP transistor 16. Further, the emitters of the first and second PNP transistors 15, 16 are coupled to each other through a common-emitter resistor 33. Therefore, the first and second power amplifiers 12, 13 of the preamplifier 11 are connected to each other in a differential amplifier configuration. The common-emitter resistor 33 further determines the gain of the balanced transformer-less amplifier circuit 10 together with the feedback resistors 31, 32.
The offset output voltage detecting circuit 14 is constituted by a differential amplifier with both an inversed and a non-inversed input terminal. The inversed and non-inversed input terminals of the offset output voltage detecting circuit 14 are connected to the first and second output signal supply terminals 28, 29, respectively. Thus, the outputs on the first and second output signal supply terminals 28, 29 are applied to the inversed and non-inversed input terminals of the offset output voltage detecting circuit 14. Here, the DC levels of the outputs on the first and second output signal supply terminals 28, 29 are offset from each other in the practical circuit structure due to unbalances between corresponding circuit elements or the like. The offset output voltage detecting circuit 14 detects the difference between the DC levels of the outputs, i.e., the offset output voltage. The output terminal of the offset output voltage detecting circuit 14 is connected to the base of the second PNP transistor 16. A prescribed terminal of the offset output voltage detecting circuit 14 is further coupled to a terminal 34. The terminal 34 is adapted for coupling to the ground potential source 27 through a capacitor 35.
As is mentioned above, the prior art balanced transformer-less amplifier circuit needs both a positive and a negative power supply voltage source, i.e., the positive and negative voltages +Vcc, -Vee. However, battery operated portable equipment should have a power supply voltage source which is as simple as possible. In this connection, the use of the two power supply voltage sources, i.e., the positive and negative voltage sources, is extremely undesirable for battery operated portable equipment.
Here, if the prior art balanced transformer-less amplifier circuit was modified so that the terminal 18 was connected to the ground potential source 27 instead of the source of the negative voltage -Vee, the prior art balanced transformer-less amplifier circuit could be operated with only the positive power source voltage +Vcc. In this case, the offset output voltage detecting circuit 14 would be able to detect the offset output voltage between the first and second output signal supply terminals 28, 29 when the offset output voltage was positive. However, the output potential of the offset output voltage detecting circuit 14 does not go below the ground potential. As a result, the offset output voltage detecting circuit 14 fails to operate when the offset output voltage is negative. Thus, the prior art balanced transformer-less amplifier circuit would fail to compensate completely for the offset output voltage, if the prior art balanced transformer-less amplifier circuit were modified, as mentioned above.